1. Field of the Invention
The present invention relates to flywheels and, more particularly, to a flywheel having an improved torque capacity, that can change vibrational behavior between two characteristics, thereby suppressing resonance and improving torque variation absorbing effects.
2. Description of the Related Art
Flywheels which include driving and driven side flywheels, a spring mechanism for connecting the driving and driven side flywheels, and a friction mechanism are known, as shown, for example, in U.S. Pat. Nos. 4,468,207, 4,274,524, 4,351,168, 2,042,570, 4,445,876, 2,729,079, 2,437,537, 4,663,983, 4,220,233, 4,002,043; GB-A-2,000,257; DE-A-2,926,012; Automotive Engineering, vol. 93, page 85; Japanese Utility Model Publications SHO 61-23542, SHO 61-23543, SHO 61-23544, SHO 59-113548, SHO 59-108848, SHO 56-6676, and SHO 56-109635; and Japanese Patent Publications SHO 61-59040, SHO 61-59024, and SHO 61-52423. Further, a vibrational system including two kinds of springs arranged in parallel to each other and a friction mechanism arranged in series with one of the two springs is shown in Shock and Vibration Hand Book, vol. 2, McGraw Hill, though it does not relate to a flywheel device.
The prior art flywheels have a single kind of vibrational characteristic generated by a single kind of spring mechanism, even if the spring mechanism itself includes a plurality of coil springs arranged in series or in parallel with each other. The single kind of vibrational characteristic causes the flywheel to have a single first mode resonance speed throughout the entire range of engine speeds. The single resonance speed is usually set lower than the idling speed of the engine. As a result, when the engine speed passes through the resonance speed during start-up or stopping of the engine, the torsional vibration of the flyhwheel will be amplified. To suppress the amplification in the torsional vibration, a continuously sliding friction mechanism (often called a hysteresis mechanism) which continuously slides throughout the entire range of engine speeds is disposed between the driving and driven side flywheels.
However, there are two problems with the above-described prior art flywheels. One problem is that a considerably large resonance remains at the resonance speed even if the friction mechanism is provided, because the characteristic of the flywheel is determined more by the spring mechanism than by the friction mechanism. The other problem is that the friction mechanism deteriorates the acceleration tansmittance rate (which corresponds to a damping characteristic of the flywheel device) at the standard range of engine speeds above the idling speed. This is because the frictional force due to the sliding friction mechanism exists over the entire range of engine speeds and because temporary sticking frequently occurs in the friction mechanism.
U.S. application, Ser. No. 07/93,573 filed Sept. 4, 1987 presents a flywheel device designed to overcome the problems of the above-mentioned flywheels. More specifically, that flywheel device has two kinds of spring mechanisms and a momentarily sliding friction mechanism so that the flywheel device has two vibrational characteristics between which the vibrational behavior of the flywheel device changes to thereby suppress the resonance without using a continuously sliding friction mechanism. That device improves the torque transmittance rate (the torque absorbing effect) at a standard range of engine speeds. However, it would be desirable to further improve the torque capacity of the flywheel device described in that application.